Crack detector for edge of moving strip which discriminates between cracks and pin-holes

ABSTRACT

A housing, positioned at an edge of a moving strip, contains a source of light disposed on one side of said strip and first and second light-sensitive means, spaced apart from each other longitudinally of said strip, disposed on the other side of said strip. A light shield, positioned adjacent said edge of the strip, comprises a shielding element having an inner edge extending beyond the edge of the strip between said strip and said light-sensitive means. First and second apertures in said shielding element, spaced apart from each other both longitudinally and laterally of said strip, permit only light from said source passing through predetermined areas of said strip to impinge on said light-sensitive means. Circuits are provided for producing an output signal indicative of a crack only when a signal produced in response to light impinging on said first light-sensitive means precedes a signal produced in response to light impinging on said second light-sensitive means by the time required for the strip to travel from said first aperture to said second aperture.

United States Patent 3,299,771 1/1967 Cauliereta].

Isadore Moskowitz [72] Inventor Baltimore, Md. [21] Appl. No. 795,511 [22] Filed Jan. 31, 1969 [45] Patented Jan. 26, 1971 [73] Assignee Bethlehem Steel Corporation a corporation of Delaware [54] CRACK DETECTOR FOR EDGE OF MOVING STRIP WHICH DISCRIMINA'I'ES BETWEEN CRACKS AND PIN-HOLES 7 Claims, 7 Drawing Figs.

[52] US. Cl 250/219, 356/237 [51] Int. Cl....'. G0ln 21/16 [50] Field ofSeareh"... 250/219; 356/200, 237

[56] References Cited UNITED STATES PATENTS 2,916,633 12/1959 Stone et al 250/219 250/219 Primary Examiner-Archie R. Borchelt I ABSTRACT: A housing, positioned at an edge of a moving strip, contains a source of light disposed on one side of said strip and first and second light-sensitive means, spaced apart from each other longitudinally of said strip, disposed on the other side of said strip. A light shield, positioned adjacent said edge of the strip, comprises a shielding element having an inner edge extending beyond the edge of the strip between said strip and said light-sensitive means. First and second aper tures in said shielding element, spaced apart from each other both longitudinally and laterally of said strip, permit only light from said source passing through predetermined areas of said strip to impinge on said light-sensitive means. Circuits are pro vided for producingan output signal indicative of a crack only when a signal produced in response to light impinging on said first light-sensitive means precedes a signal produced in response to light impinging on said second light-sensitive means by the time requiredfor the strip to travel from said first aperture to said second aperture.

l6 rz -L LIGHT SOURCE j .1 94

.96 ILIGHT SHIELDI I I I50 /04 was PHOTOMULTIPLIER PHOTOMULTIPLIER $5 ruse TUBE /52 AMP PRE-AMP PRE-AMP PUII SE I I /s4 SHAPER I40 POWER -AMP POWER-AMP I46 BISTABLE BISTABLE F T 'i SWITCH i wrrcn I56 GATE GATE SIGNAL RE-SET we PRE-SET RE-s ET COUNTER BISTABLE .RE-SET SWITCH J ,ao GATE SIGNAL GATE [62 ATE TIME I I DELAY EDGE PRE SET RE- SET SIGNAL CRACK DATA COUNTER o LIGHT OUTPUT *REcoRo SIGNALS I64 ALARM HORN PATENTED m2 6 tan SHEET 1 0f 3 INVENTOR lsaaore' Mos/row/lz V FATENTED JAN 2 6 |97| sum 2 or 3 INVENTOR /s0d0re Moslrow/fz r in:

PATENTEU JAN26 I971 SHEEI 3 OF 3 72% LIGHT SOURCE 1 z 94%; I]. I In 96 1 m ILIGHT SHIELD! l /04 -/06 PHOTOMULTIPLIER PHOTOMULTIPLIER was TUBE 58 I I J52 AMP PRE-AMP PRE-AMP PUSE I I L /54 SHAPER I40 POWER AMP POWER AMP I46 BISTABLE BISTABLE RE-SET SWITCH I SWITCH In I56 GATE GATE SIGNAL RE-SET we PRE-SET COUNTER BISTABLE RE-SET SWITCH J 0 GATE SIGNAL GATE I62 GATE I66 TIME I I DELAY EDGE PRE -SET RE- SET /70 SIGNAL CRACK DATA COUNTER LIGHT OUTPUT RECORD [64/ SIGNALS ALARM HORN INVENTOR cttnc tpsrscrolt roItEDcEoF MOVING STRIP wIIIeIInIscnIMINATEsB sTwEEN CRACKS AND PIN- HOLES" 1 BACKGROUND "or THE INVENTION This invention relates to apparatus for detecting-defects in strip,'and more particularlyyto an edge crack detector which accurately distinguishes between" edge' cracks and pinholes in moving' st'rip; I 1 Steel stripwhich hasbeen cold=rolled is generally charactetiied b'y uneven edges' anda width whichis not constant along the. entire length 'o'f'the strip. litaddition, small tears or cracks are occasibhallypreseitt iii the-edges ofth'e strip. Consequei'ttly, cold rolledj strip is generally passed through an edge-trimming machine'to provide strip of aconstant width with straight edges which are essentially'free of cracks.

Edge c'raclr's sometimes extehd'into thatportion of the strip which remains afterthe edge-trimming operation. If the strip istob'e fed subsequently. into an electroly'ti'c plating'line, it is extremely important'tha t these cracks be detected and proper i'emedialaction takenprio'rto the plating operation, as any craclt'in the edge of the "strip is likely to cause'the strip to tear across its entire width if the stiip is fed through the plating line at pr'oduction' speeds. Such remedialiaction may comprise cuttingacrack out of an'edge' of the strip. at a-point im-" mediately ahead of the plating line. The cut'is generally Crescent-shaped. to reducethe stress-atthe edge of the strip and 'thereby decrease the probability of the strip tearing thereacross, andnecessita'tessul'istantially decreasing the "speed of the strip ddring 'the making of the cut; Slowing. down the strip, rifc'ourse, increases the costs of production.

l n'view oftlie likelihood ofthe tearing-if a crack passes undetected into an electrolytic tinning line, itis essential for edge" crack detectar to' accurately detect all cracks.

' Furthennore, in order to avoid slowing-down the linewhen theie is he c'rack-ihth'e strip, it is essential for an" edge crack detector to "prodiiceno signal in the absence 'of a crack. Prior, edgecrackdtectors-frequehtly produced signals' as a result of statutes inthesmp. Y v I It'isiamobjectof this invention 'to-provide an edge crack detec't'or which' accurately:distinguishes between edge crackslIg sensitive means beirig spaced apart from'each other longitudinally pfjsaid Said t'Irst and second light-sensitive meansare adapted"tofreceive only light from said source whI h passes "through firstand-second 'areasfof said strip,

respectively, saidsecondar'eabeingspaced apart from said firstjareabothlongithdiiially -and laterally of said'strip. Means are p'rovided for producinga firstsignal "indicatin'gith'at light frorii 'said sourcehas impinged on said first light-sensitive meahs and a second signal indicatingthat light from said source has impinged on said second light-sensitive means.

FiIi-th'e r means-is pro 'ded for producing anoutput signal in-' dicative er "a crack "only when saidtirstf signal precedes said second gnalbythetiniereqtiited for'thestrip totravel from i'st area'to said-steam a're'a.

mawsssenmmmr THE DRAWINGS "0. Tim side elevatio'nal view of the apparatus of'the inr "ven on,"'parti ally cut*iiway toshow features thereo'f.

. 1' BIG? 2 is art end elevational viewofthe'a'pparatils shown in FIG 1. a v 3isa sectionalview, takenalong the'lines3 3ofEIG. 2,, agranimatic allyfshowing the salient features of the hat sensitive controlmeahs'forroughpositioning ofthe' subject apparatus relative-titan edge ofa' moving stn'p.

. I654 is a'diagrammatic isometricview showing the'general. I

' 'al relationship of the'salie'nt features ofthe invention.

FIG. 5 is a plan view diagrammatically showing the drive system for fine positioning the subject apparatus relative to a strip edge, while FIG. 6 is a side elevational view of the same.

FIG. 7 is a block diagram of the electrical circuits of the invention.

DESCRIPTION OF THE PREFERRED-EMBODIMENT As shown in FIGS. 1 and "2, a U-shaped housing 10 comprise ing an upper section '12 and a lower section 1 4 is disposed with portions of said upper and lower'sections extending inwardly of the edge of'a moving strip 16. The housing 10 is secured to a carriage 18 which is adapted to move transversely with respectto the path of the' strip 16. The carriage 18 comprises an upper-plate 20which is spaced from a lower plate 22 by bolts 24welded to said'upper plate 20 which threadedly engage cooperating traveling nut pairs 26 whereby-the spacing between said upper and said lower plates can be varied.

Secured to the lower plate 22 by said bolts 24 and traveling nut pairs 26 are oppositely disposed angle irons 28 on which wheels -30are rotatably mounted. Depending from the rear of the'lower plate 22 is a vertical plate 32 to which a traveling nut 34issecured. f

A housing 36 is proyided with means for driving and guiding the carriage l8 transversely relative to the direction of strip travel. Said housing36 comprises a bottom channel 38 and side plates 40. Secured to-eachside plate 40 is a pair of Iongitudinally disposed angle irons 42 which receive the carriage wheels 30. A longitudinal guide 'strip 44 on the lower angle irons 42 prevents lateral movement of the carriage l8.

Mounted on the bottom'channel 38'is a motor 46 which is" connected through a coupling 48 to a screw 50 supported at each end by bearings 52. The traveling nut 34 threadedly engages the screw 50 whereby rotation of said screw drives the they are-well known in the art, which cause the motor 46 -to drive the carriage 18 inwardly of the edge of the strip 16.until substantially no light from the lamp 58 impinges on photosensitiv'e means 66'. Should the strip 16 subsequently become narrower, thereby resulting in the illumination of means 66 by lamp 58,'said circuits cause the motor 46 to drive the carriage '18"fu'rther inwardly until again there is no light from lamp 58 impinging'on means 66. Should the strip subsequently become sufficiently wider whereby substantially no light from lamp v60 impinges'on photosensitive means 68, said circuits cause the motor 46"to drive the carriage 18. outwardly until light from lamp again impinges on means 68. Should no light. from lamp 62 impinge on means70, said circuits cause the carriage 18 to becompletely retracted to its outermost position.

The above description relates to means for rough positioning a housing l0 relative to an edge of a moving strip 16.

- 76, respectively. Thepair of lamps 74 is spaced from the pair of lamps 76'longitudinally of the strip 16. The upperisection 1'2 is .provided with windows 78 and 80 disposed directly belo'w'thepairs of lamps 74 and 76, respectively. For reasons whichwill'become clear as the description proceeds, the wihdow 78 is disposed closer to the front edge of the'upper section 12 of the housinglo than is the window 80.

Disposedin the'top of-thelower section 14 of thehousing 10 is a pair of windows 82 and 84 in'vertical-registry with the windows 78 and 80, respectively. The windows 82 and 84 comprise ultraviolet transmitting filters, thereby decreasing the probability that light from sources other than the ultraviolet light source 72 will enter the lower section 14. Housed in the lower section 14 in vertical registry with the windows 82 and 84 are first and second light-sensitive means, respectively, comprising photomultiplier tubes 86 and 88. Preferably, Fresnel lenses 90 and 91 are disposed between said windows and said photomultiplier tubes for focusing any light passing through the windows 82 and 84 on the active areas of the photomultiplier tubes 86 and 88.

A light shield 92, the drive system for which will be discussed later in connection with FIGS. and 6, is interposed between the upper and lower sections of the housing 10, and is adapted to be driven along the top of the lower section 14 of said housing transversely of the direction of strip travel. The light shield 92 broadly comprises an upper shielding element 94, a lower shielding element 96, and a vertical plate 98 connecting said elements at the rear ends thereof. Mounted in the lower side of the upper shielding element 94 is a small source of light 100 which cooperates with a photosensitive cell 102 mounted in the upper side of the lower shielding element 96 to control the drive system for properly positioning the light shield 92 relative to the edge of the strip 16. The control circuit, not shown, may be similar to that disclosed in US. Pat. No. 3,263,086 to D. R. Brosious et al., issued Jul. 26, 1966, which utilizes the light impinging on a photosensitive cell mounted on a light shield to automatically maintain constant the position of said light shield relative to an edge of a moving strip.

The upper and lower shielding elements 94 and 96 are each provided with first and second apertures 104 and 106 therein, the apertures in the upper shielding element 94 being in vertical registry with those in the lower shielding element 96. During operation of the subject invention, the apertures 104 are adapted to be disposed between the windows 78 and 82 at all times, while the apertures 106 are adapted to be disposed between the windows 80 and 84 at all times.

It should be noted that the outer edges of apertures 106, i.e. the edges closest to the edge of the strip, are disposed closer to said edge than are the outer edges of the apertures 104. In the embodiment shown, said apertures are one-quarter inch square, the outer edges of apertures 106 are disposed threeeights of an inch from the edge of the strip, and the outer edges of apertures 104 are disposed three-quarters of an inch from said edge of the strip. Thus, there is a gap of one-eighth of an inch between the inner edges of apertures 106 and the outer edges of apertures 104 relative to the edge of the strip.

The outer edges of apertures 106 are disposed three-eights of an inch inwardly of the edge of the strip because the strip will subsequently be passed through an edge-trimming machine'which removes three-eights of an inch from each edge of the strip. Thus, there is no need to inspect this portion of the strip. The apertures 106 are spaced from apertures 104, relative to the edge of the strip, so that only light from said source which passes through a first area of said strip will impinge on the photomultiplier tube 86, while only light from said source which passes through a second area of said strip will impinge on the photomultiplier tube 88. By reason of said first area being spaced apart from said second area both longitudinally and laterally of said strip, cracks deeper than threequarters of an inch, e.g. crack 105, will be detected by the circuits of the invention while pinholes will not. How this is accomplished will become clearer as the description proceeds.

The drive system for the light shield 92 is shown diagrammatically in FIGS. 5 and 6. As shown, a rack 108 is secured by I a bracket 110 to the vertical plate 98 connecting the upper and lower shielding elements 94 and 96, respectively. .Rotatably mounted on the side of the rack 108 is a roller 112 a pinion connected to the shaft of a motor 122 mounted in said lower section 14 and controlled by the aforementioned control circuits. The rack 108 is also provided with a stud 124 which is adapted to trip limit switches 126 and 128 which control the extent of motion of the light shield 92 within the housing 10. The mechanical stops 116 prevent damage to the strip 16 and the light shield 92 in the event of a malfunction of the limit switches 126 and 128. "P

The lower shielding element 96 is provided with rollers-130 which rotatably engage the top of the lower section 14 of the housing 10. ln addition, said element 96 is provided with a cut.

out section 132 having longitudinal guide strips 134 which engage rollers 136 mounted on the top of the lower section 14 of the housing 10.

FIG. 7 shows a control circuit which produces an output signal in response to an edge crack deeper than three-quarters of an inch but which produces no output signal when a pinhole passes between either of the pairs of apertures 104 or 106. Light passing through the apertures 104 impinges on the photomultiplier tube 86 and producesan output signal which is amplified by a preamplifier 138. The signal is then supplied to a power amplifier 140 having a low output impedance which matches the impedance of the digital logic circuits to which the signal is subsequently supplied. The output from the power amplifier 140 is supplied to a bistable switch 142 which produces an output gate signal.

Similarly, light passing through the apertures 106 impinges on the photomultiplier tube 88 and produces an output signal which is amplified by a preamplifier 144 and a power amplifier 146 having a low output impedance. Said signal is then supplied to a bistable switch 148.

Disposed immediately ahead of the light shield 92 is a stripdriven pulse tachometer 150. In the embodiment shown, the tachometer 150 produces 24 pulses per foot of strip travel. The output from the tachometer 150 is amplified by an amplifier 152, modified by a pulse shaper 154, and supplied to a gate 156.

In the absence of a signal from the bistable switch 142, the

gate 156 is in its closed state; hence, none of the pulses from the pulse shaper 154 passes therethrough. However, when there is a gating signal from thebistable switch 142, as a result of light impinging on the photomultiplier tube 86, the pulses from the pulse shaper 154 pass through the gate 156 and initiate a preset counter 158 which is adapted to actuate a bistable switch 160 after a predetermined number of pulses. The number of pulses for which the counter 158 is set should correspond to the distance between the apertures 104 and 106. In the instant case, the center lines of said apertures are spaced 5 inches apart; hence, the counter 158 should be set for 10 pulses.

The output from the bistable switch 160 provides a gating signal which is supplied to a gate 162 connected to the output of the bistable switch 148. In response to the gating signal from the bistable switch 160, the gate 162 opens, thereby permitting a signal from the bistable switch 148 to pass therethrough and actuate output means 164 indicating that an edge crack deeper than three-quarters of an inch has passed through the light shield 92. The signal from the output means 164 may be' utilized to mark a data record, sound a horn, illuminate asignal light, etc., whereby the approximate position of the edge crack may be recorded. 1 if The signal from the bistable switch 160 is also utilized to open a gate 166 to which pulses from the pulse "shaper 154 are supplied when the gate 156 is open. Said pulses pass through the gate 166 and initiate a preset counter 168 which is adapted to return the subject circuit to its initial condition, i.e., its condition prior to the passage of light through the apertures 104, a short time interval after the bistable switch 160 has been actuated. This time interval permits the strip to travel a few, e.g. 2, inches while the gate 162 is open, thereby allowing for'some .error in the calibration of ,the, tachometer 150. To permit the strip to travel 2 inches while the gate 162 is open, the counter 168 is preset for four pulses, after which a signal therefrom"- resets bistable switches 142', 148 and 160 and preset counter 158. Said signal also passes througha time delay 170 and subsequently resets preset counter 168. The time delay 170 allows sufficient time for the output signal from the preset counter 168 to reset the necessary circuits before the counter 168 itself is reset.

The above description of the-subject circuit sets forth the operation thereof when an edge crack passes through the light shield 92. The following will set forth the operation thereof when a pinhole passes therethrough. I

Inasmuch as the apertures 104 and 106 arespaced apart from each other laterally of the strip 16, a pinhole can never pass between both the apertures 104 and the apertures 106. If a pinhole passes between the apertures 104, the photomultiplier tube 86 will be actuated and the subject circuit will function, as above described, to open the gate 162 However, the bistable switch 148 will produce no output signal to pass through the gate 162 and actuate the output means 164, inasmuch as said pinhole did not subsequently pass between apertures 106 a predetermined time after passing between apertures 104. If a pinhole passes between the apertures 106, the photomultiplier tube 88 will be actuated and the subject circuit will function, as above described, to supply an output signal from the bistable switch 148 to the gate 162. However, said signal cannot pass through the gate 162 inasmuch as said gate is closed since said pinhole did not pass between the apertures 104 a predetermined time prior to passing between the apertures 106.

1 claim:

1. Apparatus for detecting cracks in an edge of a moving strip, comprising:

a. a source of light disposed on one side of said strip;

b. first light-sensitive means, disposed on the otherside of said strip, adapted to receive only light from said source which passes througha first area of said strip;

c. second light-sensitive means, disposed on the other side of said strip, adapted to receive only light from said source which passes through a second area of said strip, said second area being spaced apart from said first area both longitudinally and laterally of said strip;

d. means for producing a first signal indicating that light from said source has impinged on said first light-sensitive means; 7

e. means for producing a second signal indicating that light from said source has impinged on said second light-sensitive means; and

f. means for producing an output'signal indicative of a crack only when said first signal precedes said second signal by the time required for the strip to travel from said first area to said second area.

2. Apparatus for detecting cracks in an edge of a moving strip, comprising:

a. a source of light disposed on one side of said strip;

b. first and second light-sensitive means disposed on the other side of said strip in substantial alignment with said source of light, said light-sensitive means being spaced apart longitudinally of said strip;

c. means, interposed between said strip and said light-sensitive means, for permitting only light from said source which passes through a first and a second area of said strip to impinge on said first and second light-sensitive means, respectively, said first and second areas being spaced apart from each other both longitudinally and laterally of said strip;

d. means for producing a first signal indicating that light from said source has impinged on said first light-sensitive means;

e. means for producing a second signal indicating that light from said source has impinged on said second light-sensitive means; and i f. means for producing an output signal indicative of a crack only when said first signal precedes said second signal by the time required for the strip to travel from said first area to said second area. 3. Apparatus as recited in claim 2, m which means (0) comprises:

a. light-shielding means, positioned adjacent said edge of said strip, comprising a shielding element having an inner edge extending beyond said edge of said strip between said strip and said light-sensitive means; and

b. first and second apertures in said shielding element, said apertures being adapted to permit only light from said source which passes through said first and second areas, respectively, to impinge on said first and second light-sensitive means, respectively.

4. Apparatus as recited in claim 3, in which said light-shielding means further comprises a second shielding element, disposed between said source of light and said strip, having first and second apertures therein in registry with the apertures in the shielding element extending between said strip and said light-sensitive means. 7

5. Apparatus as recited in claim 1, in which means (f) comprises:

a. gating means, connected to the output of means (e) adapted to be opened by a control signal supplied thereto;

b. means, connected to said gating means, for producing a signal indicative of a crack only when a signal from means (e) passes through said gating means;

c. means for producing pulses at a rate proportional to the speed of said strip; and

d. means for counting said pulses only when the signal of means (d) is produced, and for supplying a control signal to said gating means whereby said gating means is opened for a short time interval after a predetermined number of said pulses have been counted.'

6. Apparatus as recited in claim 4, in which means is provided for maintaining the position of said light-shielding means constant relative to said edge of said strip.

7. Apparatus for detecting cracks in an edge of a moving strip, comprising:

a. a housing positioned adjacent said edge and adapted to move transversely with respect to the direction of strip movement, said housing comprising a first section which extends inwardly of said edge on one side of said strip and a second section in registry with said first section on theother side of said strip;

b. a source of light disposed in said first section;

c. first and second light-sensitive means disposed in said second section in substantial alignment with said source of light, said light-sensitive means being spaced apart from each other longitudinally of said strip;

d. light-shielding means, positioned between said first and second sections of said housing and adapted to move in said housing transversely with respect to the direction of strip movement, comprising first and second shielding elements, in substantial registry, having an inner edge extending between said source of light and said strip and said light-sensitive means and said strip, respectively;

e. first and second apertures in each of said shielding elements, said apertures being disposed in substantial alignment with said source of light and said first and said second light-sensitive means, respectively, and being spaced apart from each other both longitudinally and laterally of said strip;

f. means connected to said light-shielding means for maintaining the position thereof constant relative to said edge of said strip;

g. means for producing a first signal indicating that light has impinged on said first light-sensitive means;

h. means for producing a second signal indicating that light has impinged on said second light-sensitive means; and

i. means for producing an output signal indicative of a crack only when said first signal precedes said second signal by the time required for the strip totravel from said first aperture to said second aperture. 

1. Apparatus for detecting cracks in an edge of a moving strip, comprising: a. a source of light disposed on one side of said strip; b. first light-sensitive means, disposed on the other side of said strip, adapted to receive only light from said source which passes through a first area of said strip; c. second light-sensitive means, disposed on the other side of said strip, adapted to receive only light from said source which passes through a second area of said strip, said second area being spaced apart from said first area both longitudinally and laterally of said strip; d. means for producing a first signal indicating that light from said source has impinged on said first light-sensitive means; e. means for producing a second signal indicating that light from said source has impinged on said second light-sensitive means; and f. means for producing an output signal indicative of a crack only when said first signal precedes said second signal by the time required for the strip to travel from said first area to said second area.
 2. Apparatus for detecting cracks in an edge of a moving strip, comprising: a. a source of light disposed on one side of said strip; b. first and second light-sensitive means disposed on the other side of said strip in substantial alignment with said source of light, said light-sensitive means being spaced apart longitudinally of said strip; c. means, interposed between said strip and said light-sensitive means, for permitting only light from said source which passes through a first and a second area of said strip to impinge on said first and second light-sensitive means, respectively, said first and second areas being spaced apart from each other both longitudinally and laterally of said strip; d. means for producing a first signal indicating that light from said source has impinged on said first light-sensitive means; e. means for producing a second signal indicating that light from said source has impinged on said second light-sensitive means; and f. means for producing an output signal indicative of a crack only when said first signal precedes said second signal by the time required for the strip to travel from said first area to said second area.
 3. Apparatus as recited in claim 2, in which means (c) comprises: a. light-shielding means, positioned adjacent said edge of said strip, comprising a shielding element having an inner edge extending beyond said edge of said strip between said strip and said light-sensitive means; and b. first and second apertures in said shielding element, said apertures being adapted to permit only light from said source which passes through said first and second areas, respectively, to impinge on said first and second light-sensitive means, respectively.
 4. Apparatus as recited in claim 3, in which said light-shielding means further comprises a second shielding element, disposed between said source of light and said strip, having first and second apertures therein in registry with the apertures in the shielding element extending between said strip and said light-sensitive means.
 5. Apparatus as recited in claim 1, in which means (f) comprises: a. gating means, connected to the output of means (e) adapted to be opened by a control signal supplied thereto; b. means, connected to said gating means, for producing a signal indicative of a crack only when a signal from means (e) passes through said gating means; c. means for producing pulses at a rate proportional to the speed of said strip; and d. means for counting said pulses only when the signal of means (d) is produced, and for supplying a control signal to said gating means whereby said gating means is opened for a short time interval after a predetermined number of said pulses have been counted.
 6. Apparatus as recited in claim 4, in which means is provided for maintaining the position of said light-shielding means constant relative to said edge of Said strip.
 7. Apparatus for detecting cracks in an edge of a moving strip, comprising: a. a housing positioned adjacent said edge and adapted to move transversely with respect to the direction of strip movement, said housing comprising a first section which extends inwardly of said edge on one side of said strip and a second section in registry with said first section on the other side of said strip; b. a source of light disposed in said first section; c. first and second light-sensitive means disposed in said second section in substantial alignment with said source of light, said light-sensitive means being spaced apart from each other longitudinally of said strip; d. light-shielding means, positioned between said first and second sections of said housing and adapted to move in said housing transversely with respect to the direction of strip movement, comprising first and second shielding elements, in substantial registry, having an inner edge extending between said source of light and said strip and said light-sensitive means and said strip, respectively; e. first and second apertures in each of said shielding elements, said apertures being disposed in substantial alignment with said source of light and said first and said second light-sensitive means, respectively, and being spaced apart from each other both longitudinally and laterally of said strip; f. means connected to said light-shielding means for maintaining the position thereof constant relative to said edge of said strip; g. means for producing a first signal indicating that light has impinged on said first light-sensitive means; h. means for producing a second signal indicating that light has impinged on said second light-sensitive means; and i. means for producing an output signal indicative of a crack only when said first signal precedes said second signal by the time required for the strip to travel from said first aperture to said second aperture. 